Stopped-flow kinetic studies of the formation and disintegration of polyion complex micelles in aqueous solution

Abstract

The formation of soluble polyion complexes (PICs) from anionic block copolymers, poly(ethylene oxide)-b-poly(sodium 4-styrene sulfonate) (PEO-b-PSSNa) and cationic block copolymers, poly(ethylene oxide)-b-poly(quaternized 2-(dimethyl amino)ethyl methacrylate) (PEO-b-PQDMA) was investigated by fluorescence spectroscopy, laser light scattering (LLS), and stopped-flow light scattering. Colloidally stabilized dispersions could be obtained upon direct mixing of the aqueous solutions of these two block copolymers, which indicated the formation of core-shell nanostructures with the core consisting of interpolymer electrostatic complexes between PSSNa and PQDMA blocks and the corona of PEO block. Both LLS and fluorescence results revealed that the most compact complex micelles formed at the equal molar ratio of oppositely charged SSNa and QDMA residues. The kinetics of the assembly process was studied via stopped-flow upon direct mixing of the two polymer solutions. The complexation process between PEO-b-PQDMA and PEO-b-PSSNa was fast and could finish within seconds. Moreover, the relaxation process can only be detected at near equal SSNa to QDMA molar ratios. The relaxation curves can be well fitted by a double-exponential function, leading to a fast relaxation process related to the initial quasi-equilibrium complex formation and a slow process related to the pre-complex structure rearrangements to the final equilibrium complexes. Both stages are determined as second-order reactions and processed through a micelle fusion-fission mechanism. Fluorescence kinetic studies revealed that the neutralization of an oppositely charged polyion was too fast to be detected and should be completed within the stopped-flow dead-time. Thermodynamic studies revealed that spontaneous complexation is entropy driven. Upon increasing the ionic strength of the solutions, the complexation processes became slower due to the decrease of entropy driving force. The PIC dissociation process was further studied and considered to consist of two competing processes: a second-order process depending on PIC concentration and a first-order process independent of the PIC concentration.